Well, because, George, nice girls get angry when guys take advantage of them.
6 seconds sound clip from the Back to the Future movie soundboard.
You can hear this line at 01:13:17 in the Blu-ray version of the movie.
Quote context
[...]
- Okay, let's go over the plan again. 8:55, where are you going to be?
- I'm going to be at the dance. Right.
- You're gonna be in the car with her.
- Right, okay. So, right around 9:00, she's going to get very angry with me.
- Why is she going to get angry with you?
- Well, because, George, nice girls get angry when guys take advantage of them.
- You mean you're going to go touch her on her...
- Okay. So, 9:00, you're strolling through the parking lot.
- You see us struggling in the car.
[...]